Continuously tunable lasers are well-established and are commonplace in telecommunications applications. Although telecommunications lasers operate to fixed grids (e.g., ITU grids), tunable lasers may be set up for a variety of applications and wavelength tunability is desirable to allow for correction of wavelength drift as the laser ages. Unfortunately, the requirement for full range and continuous tunability results in expensive and power-hungry electronic circuitry, most particularly due to the requirement for digital-to-analog conversion (DAC) chips.
Distributed feedback (DFB) lasers in which the gratings are built into the gain medium are being replaced by distributed Bragg reflector (DBR) lasers, particularly where tunability is required. For a wide range of tunability a sampled grating (SG) DBR laser is typically used, in which multiple waveguide gratings are cascaded with periodic blank regions, and these gratings are known as comb gratings. Thus are created reflectivity peaks which can be tuned to the required lasing wavelength.
In an alternative design of a tunable laser, digital supermode DBRs (DS-DBRs) may be utilised. The DS-DBR design has the advantage over an SG-DBR in that no DACs are required. However, related art tunable lasers made entirely on semiconductor chips may include gratings with DACs for control. Thus, there is a need for tunable lasers based upon the DS-DBR design principle but with cheaper and lower power consuming control electronics, in particular not requiring DACs.
Gratings can be tuned thermally, for example by incorporating electrodes to heaters on chip. However, for speed of operation, tuning by current injection can be used, for example by biasing a p-i-n or p-n diode junction.
The gratings of tunable lasers in related art III-V semiconductor material systems may have vertical p-i-n diode junctions with a common ground or negative electrode. In these lasers multiple grating sub-regions, such as those in a DS-DBR, require separate drive contacts but must share a common ground.